Little Volcanoes -

Where Will They Show Up Next?

(Published in The EarthSphere Blog – Cover Image: Beacon Rock, The Last of Its Kind; ArcheanArt)

Driving into Portland, Oregon, from the east takes you through the small town of Boring and then across a terrain of knobby, rounded, small to medium size hills. You could be forgiven for thinking you were traversing a landscape carved over millions of years by erosion from streams and rivers, but you would be wrong. The scenic, hilly drive from Boring to Portland winds through dozens of little volcanoes.

Some Boring volcanoes reside within the Portland city limits, and the USGS still considers the Boring Volcanic Field to be an active volcanic center. Names like Mount Tabor and Mount Scott are familiar to the locals, with urban neighborhoods nestled in on the slopes of these little volcanoes.

Ironically these miniature igneous formations, described as vents and cinder cones, rest in the shadow of Mount Hood. Towering to over 11,000 feet, Mount Hood is one of the giant granddaddies of the Cascade volcanic chain. Its brother, Mount St. Helens, spectacularly exploded in 1980 when it ejected its entire peak, leaving the flat-topped mountain we see today.

Monsters like Mt. Hood build to their impressive height through continual eruptions over hundreds of thousands of years. Mount Hood has been at it for about 500,000 to 700,000 years. But the smaller Boring volcanoes are monogenetic, representing a one-shot effort where a single feature erupts, builds to between 600 and 1,000 feet, and then falls silent, waiting for the next outpouring of magma to form nearby. In total, the Boring volcanic field consists of over 80 separate vents and cones.

The Boring Volcanic Field

The Boring volcanoes are mostly located in and on the edges of the Portland Basin. They have a long history with several distinct episodes of eruptions. The earliest activity stretches back in time to between 2.6 and 2.4 million years ago, when eruptions occurred about 20 miles south of the present-day city of Portland.

After almost a million-year hiatus, the activity picked up again at 1.6 million years and expanded to encompass the full area of the modern volcanic field. The youngest volcano in the field is Beacon Rock, located on the banks of the Columbia River. This cinder cone formed a mere 57,000 years ago, but today only the central igneous plug remains.

Originally, tephra (cinder) surrounded the central plug, giving the whole structure a classic conical shape. But 15,000 years ago, the Missoula floods raged through the Columbia River Gorge, with water levels up to 1,000 feet higher than today. The force of the water ripped the tephra away from the central plug, and today it stands as an 848-foot tall cylindrical beacon welcoming visitors into the Columbia River Gorge Scenic Area.

The origin of the Boring Volcanic Field traces back to the Cascadia subduction zone, where the Juan de Fuca oceanic plate is subducting below the North American crust. As the Juan de Fuca slab descends toward the mantle, heat and pressure create reservoirs of magma that seek to move upward towards the earth’s surface. When we drive from Boring to Portland, we observe the unique topography where time and time again, magma reached the surface and spewed out to form the little volcanoes of this region. But Portland is not alone in having a broad volcanic field crowded with the remnants of monogenetic volcanoes.

Anywhere, Any Time

A recent article in Newsweek laid down the claim that new volcanoes could erupt anywhere in the American Southwest at any time. The article focused on monogenetic volcanoes like those in the Boring Volcanic Field and pointed out that about 2,200 of these features are known in the Southwest U.S. and just across the border in the Mexican State of Senora.

These Southwest volcanoes were emplaced over the past 2.6 million years, so on average, one sprang up every 1,170 years. It’s an impressive number of eruptions, and some have occurred as recently as 1000 years ago. But geology works in spurts, not long-term averages. Volcanic fields tend towards periods of high activity punctuated by long periods of quiescence.

Nevertheless, cities like Flagstaff, Arizona, and Albuquerque, New Mexico, are located within know volcanic fields, and one of the more recent eruptions in the region occurred near Flagstaff about 1,000 years ago. Geologist Greg Valentine, one of the co-authors of a recent paper titled “Quaternary basaltic volcanic fields of the American Southwest,” placed the chances of an eruption in his study area during the next 100 years at about 8 percent.

The claim of an eruption occurring “anywhere at any time” needs to be tempered with two observations. Firstly, eruptions will only occur in an active volcanic field, and the entire Southwest is not a single, huge volcanic field. The second observation is that one eruption event every 1,000 years does not constitute a high-probability risk for most people.

Next time you are gazing at Mount Hood, Mount Rainier, or any other monster volcano in the American West, remember their little-volcano brothers and sisters constantly popping their heads up here and there across the region.

The Most Dangerous Volcanoes of the Pacific Northwest — Living with the fury of Mother Earth (by WM House; Medium)